Miscibility of Tremolite and Hornblende in Progressive Skagit Metamorphic Suite, North Cascades, Washington

Misch, Peter; Rice, Jack M.

doi:10.1093/petrology/16.1.1

Cited by 45 publications

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“…Yet whilst the total number of cations may vary between 15 -16 it is possible to avoid the criticisms of Misch and Rice (1975) concerning the effect of this variability upon recalculation of ferrous and ferric iron by disjunctive alternatives: either I) if the sum of potassium, sodium and calcium ions is less than two within the unit cell formula, then the recalculation is performed with a cation sum of 15; or 2) if the sum of potassium, sodium and calcium within the unit cell formula is greater than two then the recalculation is performed for a ~3 cation unit cell for which the Na+Ca § content is ignored. The major assumption required for this procedure is that the larger calcium, sodium and potassium ions can nevEr be located on the smaller M ,M and M 3 sites.…”

Section: Discussionmentioning

confidence: 99%

Compositional zoning in sodic amphiboles from the blueschist facies

Wood¹

1980

Mineral. mag.

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SUMMARY. The sodic amphiboles possess two independent chemical substitution series (Fe 3 +-A1 and Fe z + Mg) that combine to provide a 'plane' of compositions. Yet at no single T and P are compositions covering the whole plane stable: (i) pure riebeckite exists under low-P conditions but breaks down in normal blueschists to give deerite; (ii) ferro-glaucophane is in competition at all except the lowest blneschist temperatures with almandine garnet; (iii) magnesio-riebeckite is stable at high-T and low-P but within the blueschist facies is replaced by the alternative higher density aegirine-talc assemblage; and (iv) glaucophane is stable only at high-P.At higher T and P than those of the blueschists, competition from NaCa pyroxenes, garnets, and deerite first erodes, and then removes, nearly all sodic amphibole compositions. At low-P the normal sodic amphiboleforming reactions from stilpnomelane and chlorite (in the presence of iron oxides, albite, etc.) produce an initial 'riebeckitic' amphibole that subsequently becomes more glaucophanitic with increasing P. Under certain conditions, perhaps connected with hydrous fluid overpressures, these reactions become transposed such that crossitic compositions become replaced whilst ferroglaucophane to glancophane compositions remain stable.

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Section: Discussionmentioning

confidence: 99%

Compositional zoning in sodic amphiboles from the blueschist facies

Wood¹

1980

Mineral. mag.

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“…Although there is much discussion in the literature about the presence (e.g., Robinson et al, 1981) or absence (e.g., Grapes and Graham, 1978) of a calcic amphibole solvus, experimental evidence suggests a well-defined solvus (e.g., Oba, 1980), and many hornblende-actinolite pairs have been observed in nature (e.g., Klein, 1969;Cooper and Lovering, 1970;Brady, 1974;Misch and Rice, 1975;Tagiri, 1977;Graham, 1974;Sampson and Fawcett, 1977). Although the shape and exact temperature of the crest of the solvus are not known, a stylized solvus has been constructed (using the cited data; after Robinson et al, 1981) and is shown as Figure 3.…”

Section: Other Amphibole Reactionsmentioning

confidence: 99%

Hydrothermal Alteration of Peridotite from the Galicia Margin, Iberian Peninsula

Kimball¹,

Evans²

1988

Proceedings of the Ocean Drilling Program

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The Galicia margin lies northwest of the Iberian Peninsula and is a passive ocean margin with thin sedimentary cover. Altered peridotite was recovered from ODP Site 637, on the north-trending ridge at the western edge of the margin, near the oceanic/continental crust boundary.The altered ultramafics were originally clinopyroxene-rich upper mantle harzburgites and are now extensively serpentinized (>85%) and cut by very late-stage carbonate veins. Despite pervasive late, low-temperature alteration, evidence of early, high-temperature alteration remains. Alteration is apparent as (1) amphibole rims on clinopyroxene (>800°C), (2) hornblende + tremolite (450° to 800°C), (3) breakdown of hornblende to form tremolite + chlorite (<450°C), (4) zoned Cr-spinels, (5) hydration of orthopyroxene and olivine to serpentine, (6) serpentine veins, (7) replacement of pyroxene and olivine by calcite, and (8) calcite veins and vugs.Both the relict igneous and the high-temperature alteration minerals (amphiboles) show evidence of brittle deformation. Subsequent low-temperature alteration veins and minerals are deformed only in faulted and brecciated zones. This textural evidence suggests that the low-temperature alteration occurred after emplacement of the ultramafics at the surface. Serpentine fills tension fractures in orthopyroxene, and both serpentine and calcite fill tension cracks in olivine.The high-temperature alterations in these samples are similar to those found in oceanic fracture zone and ophiolite ultramafics. This widespread occurrence of high-temperature alteration suggests that hot fluids were pervasive in these ultramafic blocks.Localization of high-temperature alteration close to large carbonate veins suggests channelization of the late, lowtemperature fluids. Earlier hydrations (e.g., high-temperature alterations and serpentinization) were pervasive.

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“…At moderate to high pressures, compositions evidently range continuously from tremolite-actinolite to aluminous, alkalic hornblendes (Ernst, i972;Graham, 1974) but under some lower-pressure regimes the pair actinolite +hornblende exists--suggesting the presence ofa solvus (Ernst, 1968 , pp. 27-32;Klein, 1969;Cooper and Lovering, 197o;Cooper, 1972;Brady, 1974;Misch and Rice, 1975;Tagiri, 1977; Allen and Goldie, i978 ). In contrast, Grapes and Graham (1978) conclude that most actinolite-hornblende pairs result from disequilibrium recrystallization, especially the abrupt appearance or disappearance of phases participating in amphibole-generating or amphibole-consuming reactions.…”

Section: Compositions Of Coexisting Sodic and Calcic Amphibolesmentioning

confidence: 99%

Coexisting sodic and calcic amphiboles from high-pressure metamorphic belts and the stability of barroisitic amphibole

Ernst

1979

Mineral. mag.

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SummaryCompositions of glaucophanes and actinolite-hornblende solid solutions occurring in chemically similar metabasaltic rocks from blueschist terranes in east-central Shikoku, W. California, Valtournanche (W. Alps), and W. Liguria are compared. Chemical contrasts among coexisting Na and Ca amphibole pairs, which reflect disparate P-T histories under the presumed attendance of local equilibrium, include: Na contents are rather high among barroisitic hornblendes from the western and Ligurian Alps, as well as among high-grade tectonic blocks from California; in contrast, actinolitic amphiboles from both lower-grade Franciscan tectonic blocks and in situ schists and the blueschists of Shikoku are impoverished in Na relative to blue-green hornblendes. Sodic amphiboles contain less than 0. 5 Aliv per formula unit, whereas Alvi is very high; a situation reversed among calcic amphiboles. The Na + Ca contents ofglaucophanes are strongly clustered around the sum of 2.0 (i.e. A site vacant) whereas calcic amphiboles have a wider range with the A site variably occupied. No solvus has been detected within either sodic or calcic amphiboles under blueschist facies conditions. For low-grade metabasaltic parageneses, a miscibility gap separates these two amphibole groups; at relatively high grade such compositions have sodic calcic amphiboles of barroisitic type; this may mean that glaucophane + hornblende assemblages are metastable, accounting for textural relations indicating that the sodic amphibole typically did not grow at the same time as the barroisite. Ti, Mn, and K appear to be concentrated in calcic amphibole compared to coexisting glaucophane, probably in the M2, M4, and A sites, respectively.Contrasts in coexisting amphibole tie lines are thought to be a consequence of the fact that the parageneses of Shikoku and California reflect high P and very high P prograde P-T paths respectively, whereas those from Valtournache and W. Liguria show evidence of decompression recrystallization (or back-reaction) of high P (i.e. eclogitic) protoliths. Comparison of the inferred physical conditions operating during the production of these four contrasting paragenetic sequences allows the provisional assignment of a P-T stability region for barroisitic amphibole in metabasaltic rocks as: P 4–5 kb at c. 350°; P 5–7 kb at c. 450 °C.

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Miscibility of Tremolite and Hornblende in Progressive Skagit Metamorphic Suite, North Cascades, Washington

Cited by 45 publications

References 0 publications

Compositional zoning in sodic amphiboles from the blueschist facies

Compositional zoning in sodic amphiboles from the blueschist facies

Hydrothermal Alteration of Peridotite from the Galicia Margin, Iberian Peninsula

Coexisting sodic and calcic amphiboles from high-pressure metamorphic belts and the stability of barroisitic amphibole

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